This invention relates to hybrid circuits for providing an interface between a pair of unbalanced unidirectional transmission paths and a balanced two wire, bidirectional transmission path. More particularly, this invention relates to hybrid circuits of such type which are direct coupled, i.e. transformerless.
Hybrid circuits are known for providing signal conversion between a balanced two wire, bidirectional transmission path and a pair of unbalanced unidirectional transmission paths, the latter comprising a first unidirectional transmission path for carrying incoming electrical signals and a second undirectional transmission path for carrying outgoing electrical signals. Such hybrid circuits are typically employed in the telephone art to provide a signal interface between a telephone carrier channel having the aforementioned pair of unidirectional transmission paths and the balanced bidirectional transmission paths at either end of the carrier channel to which are connected the subscriber loop and telephone hand set at one end, and the central office equipment at the other end.
In the past, hybrid circuits have been conventionally implemented by means of transformers having a pair of loop terminals connected to the bidirectional signal loop and two transmission path terminals each connected to a different one of the unidirectional signal paths. The transformer hybrid circuit is typically designed to provide an impedance match with a nominal (i.e., statistically averaged) bidirectional loop at the loop side and also to provide an impedance match with the unidirectional signal paths at the unidirectional transmission path side in order to optimize signal transfer between the bidirectional and the unidirectional signal paths. In addition, known transformer hybrid circuits typically include a balancing impedance network at the unidirectional transmission path side to provide signal balance in order to suppress transmission of reflected echoes from the incoming unidirectional transmission path to the outgoing unidirectional transmission path. Further, the transformer hybrid is typically configured in such a manner that longitudinal mode signals presented to the hybrid loop terminals from the bidirectional loop are suppressed. Longitudinal mode signals are typically generated by adjacent power circuits and power currents in the conductive sheath which surrounds the bidirectional loop conductors and such signals are inductively coupled to the loop conductors. Unless suppressed, these longitudinal mode signals introduce objectionable noise into the information signals, which impairs the performance of the overall system. Transformer type hybrids designed in accordance with the above requirements have been found to provide the desired functional capabilities and thus have been widely used in the telephone industry in the past. However, such hybrids suffer from several inherent disadvantages. Firstly, transformers of the type required to provide minimal adequate performance have a relatively large physical size when compared to other circuit elements and thus introduce constraints on packaging and installation of hybrid units. Further, transformers must be carefully constructed in order to provide adequate longitudinal signal suppression, which increases the manufacturing cost. In addition, transformer type hybrids are typically operated with relatively large D.C. currents flowing in the primary winding of the transformer (for reasons set forth in detail below), and yet must provide adequate frequency response over the bandwidth of the signal transmission system, typically 0.3 -3 KHZ. However, in order to provide adequate low frequency response in the presence of large direct current, a large iron core is required which increases both the physical size and the manufacturing cost of the transformer.
Due to the above-noted limitations inherent in transformer type hybrids, recent efforts in this field have been directed to the design of transformerless hybrids circuits having substantially similar functional capabilities as transformer type hybrids without the attendant limitations. State of the art hybrids of this type, of which the disclosure of U.S. Pat. No. 3,849,609 is representative, employ semiconductor current controlled current sources, hereinafter designated "current mirrors", to convert incoming signals from the unidirectional transmission path to bidirectional transmission path signals, and to convert the signals originating in the bidirectional transmission path to outgoing information signals for the unidirectional transmission path. Circuits of this type possess the decided advantage of being implementable in integrated circuit form, which results in a lower manufacturing cost than that for transformer type hybrids, good reliability, and small physical size. To date, however, efforts to design a direct coupled transformerless hybrid providing signal balance for suppressing transmission of reflected echoes from the incoming unidirectional transmission path to the outgoing unidirectional transmission path, suppression of longitudinal signals originating in the bidirectional transmission path, optimum conversion between balanced signals at the bidirectional transmission path side and unbalanced signals at the unidirectional transmission path side and good low frequency response have not met with wide success. SUMMARY OF THE INVENTION
The invention comprises a direct coupled transformerless hybrid employing current mirrors which can be manufactured for a relatively low cost, and which is reliable in operation, small in physical size, and provides direct current to the bidirectional transmission path, suppression of longitudinal signals originating in the bidirectional transmission path, signal balance for suppressing reflected echoes, optimum conversion between unbalanced signals at the unidirectional transmission path side and balanced signals at the bidirectional transmission path side, and improved frequency response over the bandwidth of the signal tranmission system in which the hybride is employed.
In the broadest aspect, the invention comprises the use of current mirrors of opposite conductivity types configured in a unique manner to additively combine normal mode signals and subtractively combine longitudinal mode signals originating in a bidirectional transmission path so that the normal mode signals are supplied to an outgoing unidirectional transmission path while the longitudinal mode signals are suppressed. Further, the impedance of the novel hybrid of the invention is matched to that of a nominal bidirectional transmission path to provide good signal transfer characteristics with bidirectional transmission paths having impedances lying in the customarily encounted range to ensure compatability of the invention with existing installations.
Signal balance is provided according to the invention by injecting portions of any signals present on the incoming unidirectional transmission path into the outgoing unidirectional transmission path to cancel signals reflected to the latter. In some embodiments of the invention, means are included for providing automatic adjustment of signal balance with bidirectional transmission paths having a range of resistances, i.e. paths having different lengths.
In those embodiments adapted for use with bidirectional transmission paths comprising the subscriber loop, means are also provided for supplying D.C. current to the subscriber loop and for coupling signals from the bidirectional transmission path to a unidirectional control signal transmission path to enable such control signals to be transmitted to remote central office equipment. In those embodiments adapted for use with bidirectional loops located at the central office equipment, means are also provided for establishing a D.C. current path for enabling operation of a central office equipment relay when a D.C. voltage of either polarity is applied by the central office equipment to the hybrid loop terminals.
For a further understanding of the nature and advantages of the invention, reference should be had to the ensuing detailed description taken in conjection with the accompanying drawings.